IaaS vs. PaaS vs. SaaS

SaaS:
Software as a Services. Essentially based on the concept of renting application functionality from a service provider rather than buying, installing and running software yourself. delivering the equivalent of a complete application suite.
Some time we call it as "Storage as a Services".  Some time called as "on-demand software"


PaaS:
Platform as a Services.

IaaS:
Infrastructure as a service.

Why we need Solr then Lucene?

Lucene:
Apache Lucene is a high-performance, full-featured text search engine library written entirely in Java. It is a technology suitable for nearly any application that requires full-text search

Solr:
Solr is an open source enterprise search server based on the Lucene Java search library, Its a extended version of Lucene with XML/HTTP and JSON APIs, hit highlighting, faceted search, caching, replication, a web administration interface, SolrJ, Configurable output format., etc.